1. Field of the Invention
The present invention relates to a flow control device, for example a flow control device serving as an idling speed controller which functions to adjust the flow rate of air flowing through a by-pass passage of an intake system, in accordance with an idling speed of an internal combustion engine for a vehicle.
2. Description of the Prior Art
As a flow control device applied heretofore to an idling speed controller for an internal combustion engine for a vehicle, there is known a flow control device wherein forward and reverse rotations of the the shaft of a stepping motor are converted into a reciprocating motion of a valve member disposed in a by-pass passage formed within a housing.
In such conventional flow control device, the shaft of the stepping motor and a cylindrical guide member mounted on the outer periphery of the shaft are threadedly engaged with each other, and there is provided means for inhibiting the rotation of the guide member, whereby forward and reverse rotations of the shaft of the stepping motor are converted into a reciprocating motion of the guide member, and the aforementioned valve member, which is fixed to the front end of the guide member, is allowed to approach to and leave from a valve seat provided within the by-pass passage. With this reciprocating motion of the valve member, the flow path area of the by-pass passage increases or decreases to control the flow rate of air flowing through the by-pass passage.
In the above conventional flow control device, the valve member is usually positioned away from a bearing for rotatably supporting the foregoing shaft, so that it sometimes vibrates largely during its reciprocating motion or when an external vibration is exerted thereon. To avoid these defects, it may be effective to support the guide member at a position near the valve member by means of a bearing. In this case, however, if there is a poor portion in relative sliding between the guide member and the bearing, this portion will badly influences the motion of the valve member, resulting in deteriorated control accuracy for the flow rate of air flowing through the by-pass passage.
Further, in the case where a large amount of air flowing through the by-pass passage is required, the maximum flow rate of the air flowing through the by-pass passage is restricted by a flow resistance of the bearing which supports the guide member.